Agriculture Reference
In-Depth Information
summer months (Godwin and Miner, 1996).
Similarly, time spent by free-range cattle in or
near the stream decreased by 86% when alter-
native water sources were provided between the
feeding area and the stream and by 97% when
the feeding area was located between the water
tank and the stream (Miner et al ., 1992). Similar
dramatic reductions in time spent in stream
(Sheffield et al ., 1997), stream bank erosion
(Sheffield et al ., 1997) and loading of solids,
N and P (Sheffield et al ., 1997; Line et al ., 2000),
have been observed by others with provision of
alternative water sources.
Rotational grazing can also result in
improved water quality, even without stream
exclusion. Stream bank stability and in-stream
habitat were similar between pastures with
buffer strips and rotationally grazed pastures,
and both were significantly improved over con-
tinuous grazing (Weigel et al ., 2000). Rotational
grazing encouraged better sod development
and helped stabilize erosive stream banks better
than continuous pasture. Continuous pastures
resulted in poor habitat quality and poor fish
communities.
on reducing pollution from point sources (e.g.
industrial and municipal waste). The EPA
became active regarding the development of
TMDLs for non-point sources of pollution in
response to lawsuits filed by environmental
organizations in several states. In these lawsuits,
environmental organizations alleged that EPA
had failed to fulfil the requirements of the Clean
Water Act as they had neither pushed states to
develop TMDLs regarding non-point sources of
pollution nor taken on responsibility for devel-
oping these TMDLs themselves.
Because of these lawsuits, guidelines for
developing TMDLs were published in 1991,
almost 20 years after the TMDL programme was
mandated. Since then different approaches have
been used to develop and implement TMDLs
(Jesiek, 2007) including mass balance models,
statistical methods and computer-based simula-
tion models; the majority of states have chosen
this last approach. The selection of models
depends on the impairment, type of pollutant
and complexity of the system.
The TMDL list identifies stream segments
that are impaired with regard to pathogens,
sediments, nutrients or benthic communities
(aquatic wildlife). Development of a TMDL for a
specific pollutant (i.e. coliform bacteria, nitrates,
benthics) involves quantifying the capacity of
a specific stream segment to incorporate that
pollutant and adapt without negative impact.
Predictive modelling is then done to quantify the
allowable level of pollution for that stream
segment. Sources of the pollutant are identified,
and allowable pollution is allocated among
sources. Sources include both point sources and
non-point sources, and the allocation of the TMDL
is to be specific (i.e. to a specific facility or farm).
In developing TMDLs, factors considered
are the background sources of the pollutant
(i.e. sources of pollution not related to human
activity) and variation in loading of pollutants
throughout the year. In developing plans to
bring impaired waters into compliance with
water quality goals, options include trying to
minimize peaks in a highly variable pollutant
load, or attempting to reduce all loadings by a
specific percentage.
Once sources of pollution are identified,
load reductions must be allocated to sources.
Allocation of the load (i.e. who can continue to
load pollutants to the stream and at what levels)
Total maximum daily loads
A significant evolution in US environmental
regulations is the implementation of total maxi-
mum daily loads (TMDL) programmes. A TMDL
is a goal, a target load of a specific pollutant
that a specific stream segment can absorb on a
daily basis without impairing a beneficial use
(i.e. aquatic life, human recreational activities,
human consumption, municipal uses; USDA,
2001). A TMDL is also a plan to restore impaired
stream segments to a form that will support
these beneficial uses. Finally, a TMDL is intended
to be a process - identifying a goal for loading of
a pollutant, developing a plan to meet that goal,
implementing the plan, assessing the effective-
ness of that plan, revising and re-implementing
the plan (US EPA, 1991).
The concept of TMDLs was first laid out in
the 1972 Clean Water Act. EPA was to direct
the states to establish TMDLs for all streams
and rivers, publish lists of impaired stream seg-
ments and develop plans for returning impaired
waters to unimpaired status. Very little was done
with this through the 1980s as the EPA focused
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